J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

/pdf/ultrawide-bandgap-semiconductors-research-opportunities-and-49ev1zu2ym.pdf

Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

We have combined Fourier-domain optical coherence tomography (FD-OCT) with a closed-loop adaptive optics (AO) system using a Hartmann-Shack wavefront sensor and a bimorph deformable mirror. The adaptive optics system measures and corrects the wavefront aberration of the human eye for improved lateral resolution (~4 μm) of retinal images, while maintaining the high axial resolution (~6 μm) of stand alone OCT. The AO-OCT instrument enables the three-dimensional (3D) visualization of different retinal structures in vivo with high 3D resolution (4×4×6 μm). Using this system, we have demonstrated the ability to image microscopic blood vessels and the cone photoreceptor mosaic.

/pdf/adaptive-optics-optical-coherence-tomography-for-high-3r5d76hchu.pdf

Adaptive-optics optical coherence tomography for high-resolution and high-speed 3D retinal in vivo imaging

Sensors and Actuators A: Physical

Many applications of ultrasound for sensing, actuation and imaging require miniaturized and low power transducers and transducer arrays integrated with electronic systems. Piezoelectric micromachined ultrasound transducers (PMUTs), diaphragm-like thin film flexural transducers typically formed on silicon substrates, are a potential solution for integrated transducer arrays. This paper presents an overview of the current development status of PMUTs and a discussion of their suitability for miniaturized and integrated devices. The thin film piezoelectric materials required to functionalize these devices are discussed, followed by the microfabrication techniques used to create PMUT elements and the constraints the fabrication imposes on device design. Approaches for electrical interconnection and integration with on-chip electronics are discussed. Electrical and acoustic measurements from fabricated PMUT arrays with up to 320 diaphragm elements are presented. The PMUTs are shown to be broadband devices with an operating frequency which is tunable by tailoring the lateral dimensions of the flexural membrane or the thicknesses of the constituent layers. Finally, the outlook for future development of PMUT technology and the potential applications made feasible by integrated PMUT devices are discussed.

/pdf/piezoelectric-micromachined-ultrasound-transducer-pmut-48hf8j9o1k.pdf

Piezoelectric Micromachined Ultrasound Transducer (PMUT) Arrays for Integrated Sensing, Actuation and Imaging

This article reports on the state-of-the-art of the development of aluminum nitride (AlN) thin-film microelectromechanical systems (MEMS) with particular emphasis on acoustic devices for radio frequency (RF) signal processing. Examples of resonant devices are reviewed to highlight the capabilities of AlN as an integrated circuit compatible material for the implementation of RF filters and oscillators. The commercial success of thin-film bulk acoustic resonators is presented to show how AlN has de facto become an industrial standard for the synthesis of high performance duplexers. The article also reports on the development of a new class of AlN acoustic resonators that are directly integrated with circuits and enable a new generation of reconfigurable narrowband filters and oscillators. Research efforts related to the deposition of doped AlN films and the scaling of sputtered AlN films into the nano realm are also provided as examples of possible future material developments that could expand the range of applicability of AlN MEMS.

/pdf/piezoelectric-aluminum-nitride-thin-films-for-3cbd1c0imr.pdf

Piezoelectric aluminum nitride thin films for microelectromechanical systems

Piezoelectric micromachined ultrasonic transducers for air-coupled ultrasound applications were fabricated using aluminum nitride (AlN) as the active piezoelectric layer. The AlN is deposited via a low-temperature sputtering process that is compatible with deposition on metalized CMOS wafers. An analytical model describing the electromechanical response is presented and compared with experimental measurements. The membrane deflection was measured to be 210 nm when excited at the 220 kHz resonant frequency using a 1V pp input voltage.

/pdf/cmos-compatible-aln-piezoelectric-micromachined-ultrasonic-15hp9cy6ih.pdf

CMOS-compatible AlN piezoelectric micromachined ultrasonic transducers

This paper examines the performance of rotating microdevices incorporating a liquid bearing to couple a rotating element to a fixed substrate. Liquid bearing technology promises to significantly improve the durability and lifetime of micromechanical motors. Here, the fluid is confined between the rotor and stator using surface patterning of a hydrophobic layer. Magnetic actuation of 10 mm diameter silicon rotor is used to characterize the liquid bearing motor at rotation rates up to 1800 rpm. Bearings with fluid thickness from 20 to 200 μm are characterized. A minimum torque of 0.15 μN-m is required to initiate rotation. At rotation rates above 720 rpm, the rotor wobble is less than ±1 mrad and the bearing exhibits viscous friction with a drag coefficient of 1.2 × 10 −3  μN-m/rpm. The drag performance of the disk-type liquid bearing using H 2 O as the fluid is approximately 15 times lower than that demonstrated in a micro-ball bearing supported rotor.

Design and characterization of MEMS micromotor supported on low friction liquid bearing

The static and dynamic characteristics of a bimorph deformable mirror (DM) for use in an adaptive optics system are described. The DM is a 35-actuator device composed of two disks of lead magnesium niobate (PMN), an electrostrictive ceramic that produces a mechanical strain in response to an imposed electric field. A custom stroboscopic phase-shifting interferometer was developed to measure the deformation of the mirror in response to applied voltage. The ability of the mirror to replicate optical aberrations described by the Zernike polynomials was tested as a measure of the mirror's static performance. The natural frequencies of the DM were measured up to 20 kHz using both stroboscopic interferometry as well as a commercial laser Doppler vibrometer (LDV). Interferometric measurements of the DM surface profile were analyzed by fitting the surface with mode-shapes predicted using classical plate theory for an elastically supported disk. The measured natural frequencies were found to be in good agreement with the predictions of the theoretical model.

/pdf/characterization-of-a-bimorph-deformable-mirror-using-ma2d0tr1gg.pdf

Characterization of a bimorph deformable mirror using stroboscopic phase-shifting interferometry

The wave front corrector is one of the three key elements in adaptive optics, along with the wave front sensor and the control computer. Low cost, compact deformable mirrors are increasingly available. We have tested the AOptix bimorph deformable mirror, originally developed for ultra-high bandwidth laser communication systems, to determine its suitability for vision science applications, where cornea and lens introduce optical aberrations. Measurements of the dynamic response of the mirror to a step input were obtained using a commercial Laser Doppler Vibrometer (LDV). A computer-controlled Twyman-Green interferometer was constructed to allow the surface height of the deformable mirror to be measured using Phase-Shifting Interferometry as a function of various control voltages. A simple open-loop control method was used to compute the control voltages required to generate aberration mode shapes described by the Zernike polynomials. Using this method, the ability of the deformable mirror to generate each mode shape was characterized by measuring the maximum amplitude and RMS error of each Zernike mode shape up to the fifth radial order. The maximum deformation amplitude was found to diminish with the square of the radial order of the Zernike mode, with a measured deformation of 8 microns and 1.5 microns achieved at the second-order and fifth-order Zernike modes, respectively. This deformation amplitude appears to be sufficient to allow the mirror to correct for aberrations up to the fifth order in the human eye.

/pdf/characterization-for-vision-science-applications-of-a-4w7gcyjssy.pdf

Characterization for vision science applications of a bimorph deformable mirror using phase-shifting interferometry

This paper examines the performance of rotating microdevices incorporating a liquid bearing to couple a rotating element to a fixed substrate. Liquid bearing technology promises to significantly improve the durability and lifetime of micromechanical motors. Here, the fluid is confined between the rotor and stator using surface patterning of a hydrophobic layer. Magnetic actuation of 10 mm diameter silicon rotor is used to characterize the liquid bearing motor at rotation rates up to 1800 rpm. Bearings with fluid thickness from 20–200 microns are characterized. A minimum torque of 0.15 µN-m is required to overcome static friction and initiate rotation. At rotation rates above 720 rpm, the rotor wobble is less than ±1 mrad and the bearing exhibits viscous friction with a drag coefficient of 1.2 × 10−3 µN-m/rpm.

/pdf/low-friction-liquid-bearing-mems-micromotor-16knwl43mc.pdf

Hyunkyu Park

Papers

CMOS-compatible AlN piezoelectric micromachined ultrasonic transducers

Design and characterization of MEMS micromotor supported on low friction liquid bearing

Characterization of a bimorph deformable mirror using stroboscopic phase-shifting interferometry

Characterization for vision science applications of a bimorph deformable mirror using phase-shifting interferometry

Low friction liquid bearing mems micromotor